LNER Class A4 4468 Mallard in the context of "All caps"

The East Coast Main Line (ECML) is a 393-mile-long (632 km) electrified railway between its northern terminus at Edinburgh Waverley and southern terminus at London King's Cross. Peterborough, Doncaster, York, Darlington, Durham and Newcastle are on the line, which is a key transport artery on the eastern side of Great Britain running broadly parallel to the A1 road. The main line acts as a 'spine' for several diverging branches, serving destinations such as Cambridge, Leeds, Hull, Sunderland and Lincoln, all with direct services to London. In addition, a few ECML services extend beyond Edinburgh to serve other Scottish destinations, such as Stirling, Inverness, Dundee, or Aberdeen.

The line was built during the 1840s by three railway companies, the North British Railway, the North Eastern Railway, and the Great Northern Railway. In 1923, the Railways Act 1921 led to their amalgamation to form the London and North Eastern Railway (LNER) and the line became its primary route. The LNER competed with the London, Midland and Scottish Railway (LMS) for long-distance passenger traffic between London and Scotland. The LNER's chief mechanical engineer Nigel Gresley designed iconic Pacific steam locomotives including Flying Scotsman and Mallard, the latter of which achieved a world record speed for a steam locomotive, 126 mph (203 km/h) on the Grantham-to-Peterborough section on 3 July 1938.

A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomotive's boiler to the point where it becomes gaseous and its volume increases 1,700 times. Functionally, it is a self-propelled steam engine on wheels.

In most locomotives the steam is admitted alternately to each end of its cylinders in which pistons are mechanically connected to the locomotive's main wheels. Fuel and water supplies are usually carried with the locomotive, either on the locomotive itself or in a tender coupled to it. Variations in this general design include electrically powered boilers, turbines in place of pistons, and using steam generated externally.

Sir Herbert Nigel Gresley CBE (19 June 1876 – 5 April 1941) was a British railway engineer. He was one of Britain's most famous steam locomotive engineers, who rose to become Chief Mechanical Engineer (CME) of the London and North Eastern Railway (LNER). He was the designer of some of the most famous steam locomotives in Britain, including the LNER Class A1 and LNER Class A4 4-6-2 Pacific engines. An A1 Pacific, Flying Scotsman, was the first steam locomotive officially recorded over 100 mph in passenger service, and an A4, No. 4468 Mallard, still holds the record for being the fastest steam locomotive in the world (126 mph).

Gresley's engines were considered elegant, both aesthetically and mechanically. His invention of a three-cylinder design with only two sets of Walschaerts valve gear, the Gresley conjugated valve gear, produced smooth running and power at lower cost than would have been achieved with a more conventional three sets of Walschaerts gear.